1. Field of the Invention
The present invention relates to a system, method and terminal for making inter-frequency and inter-system measurements for controlling reliable handovers therein and therein between.
2. Description of the Prior Art
FIG. 1 illustrates “compressed mode” which is used in wideband code division multiple access (WCDMA) to make inter-frequency measurements by user equipment (UE). Compressed mode in Release 99 (3GPP TR 25.211-25.215 which describes the physical layer, 3GPP TR 25-331 which describes the radio resource control protocol, and 3GPP TR 25-133 which describes requirements for radio resource management) uses a terminal having a single receiver. The main features of compressed mode in making inter-frequency measurements are: (1) measurement times are controlled by the radio access network (RAN); (2) measurement gaps are within a frame which are typically seven milliseconds in duration during which inter-frequency measurements are made; (3) no data transmission occurs during the gaps; and (4) higher power transmission is used during the other parts of the frame to compensate for the gap. The gap is created by increasing the data transmission rate in the frame so that the data payload transmitted in the frame takes less time providing for the measurement gap.
See the 3GPP Technical Reports TR 25.212 and 25.331 for a description of compressed mode which are incorporated herein by reference in their entirety.
A main disadvantage of inter-frequency measurements using a single receiver is the requirement that the receiver has to process higher data rates in frame(s) with the gaps (frame #2 of FIG. 1) than normal data rates (frames #1 and #3 of FIG. 1). Furthermore, the data transmissions in the frame(s) with the gap are at a higher power level to accommodate the lost transmission energy in the gap period.
FIG. 2 describes a prior art system based upon Section 6.5 “Multiple Input Multiple Output [MIMO] Antenna Processing in the 3GPP Technical Report 25.848 v4.0.0 2001-03 which is incorporated herein by reference in its entirety. The diversity system 10 utilizes known multiple downlink transmit antennas for second order applications in the UTRA Release 99 Specifications. These techniques exploit spatial and/or polarization decorrelations over multiple channels to achieve fading diversity gain.
MIMO systems use multiple antennas at the station transmitter and terminal receiver which provide advantages in comparison to transmissions using conventional single antennas. If multiple antennas are used at both the transmitter and the receiver, the peak throughput is increased using a technique known as code re-use. With code re-use, each channelization/scrambling code pair allocated for the HSS-DSCH transmission can modulate up to M distinct data streams where M is the number of base station transmitter antennas. Data streams which share the same channelization/scrambling code must be distinguished based on their spatial characteristics, requiring a receiver with at least M antennas. In principal, the peak throughput with code re-use is M times the rate achievable with a single transmit antenna. Third, with code re-use, some intermediate data rates can be achieved with a combination of code re-use and smaller modulation constellations, e.g. the 16 QAM instead of 64 QAM. Compared to a single antenna transmission scheme with a larger modulation constellation to achieve the same data rate, the code re-use technique may have a smaller required Eb/No, resulting in overall improved system performance. This technique is open loop since the transmitter does not require feedback from the UE other than the conventional HSPDA information required for rate determination. Further performance gains can be achieved using closed-loop MIMO techniques whereby the transmitter employs feedback information from the UE. For example, with knowledge of channel realizations, the transmitter could transmit on an orthogonal Eigen modes eliminating the spatial multiple-access interference. See Section 5.3 of the aforementioned Technical Report TR 25.848.
The system 10 illustrated in FIG. 2 is comprised of a MIMO transmitter 21 having M antennas 25 based on FIG. 6 in the aforementioned Section 6.5 in combination with a MIMO terminal 20′ including a receiver 20 having P antennas based on FIG. 7 of the aforementioned Section 6.5. The diagram of FIG. 6 has been modified to include a receiver 12 associated with the MIMO transmitter 21. The transceiver 24 is used at a station and is controlled by a base station controller/radio network controller 14. Similarly, FIG. 7 has been modified to include a MIMO UE 20′. The UE 20′ includes a transmitter 16 and a controller 18 which are utilized with the MIMO receiver 20. The UE 20′ is coupled by a radio link 22 containing uplink and downlink radio channels to the transceiver 24 containing the receiver 12 and transmitter 21.
The transceiver 24 receives a coded high rate data stream 26 which is inputted to a demultiplexer 28. The demultiplexer 28 demultiplexes the coded high rate data stream 26 into M data streams 30. The M data streams 30 are spread by N spreading codes applied to spread data functions 32. The spread data functions 32 produce MN substreams of the M substreams outputted by the demultiplexer 28. The M substreams (m=1 . . . M) of each group are summed by summers 34 and multiplied in multiplexers 36 by scrambling code and transmitted over the Mth antenna so that the substreams sharing the same code are transmitted over different antennas. Mutually orthogonal dedicated pilot symbols are also added by the summers 34 to each antenna's common pilot channel (CPICH) to allow coherent detection. For M=2 or for 4 antennas, the pilot symbol sequences 4, respectively, two antenna STTD or 4 antenna closed-loop diversity can be used.
The UE 20′ distinguishes the M substreams sharing the same code. P antennas 41 receive the M substreams and spatial signal processing is used to decode the M substreams. For coherent detection at the UE 20′, complex amplitude channel estimates are required for each transmit/receive antenna pair. In a flat fading channel, the channel is characterized by MP complex channel coefficients. In frequency selective channels, the channel is characterized by LMP coefficients where L is the number of RAKE receiver fingers. Channel estimates can be obtained by correlating the received signals with the M orthogonal pilot sequences. Compared to a conventional single antenna receiver, the channel estimation complexity is higher by a factor of M. For data detection, each antenna is followed by a bank of filters matched to the N spreading codes. In general, there are LN despreaders 40 per antenna. For each of the MN distinct data streams, the LP corresponding despreader outputs are each weighed by the complex conjugate of the corresponding channel estimate 42 and summed together by the space time rate combiner 44. The space-time-rate combiner is a multiple antenna generalization of a conventional rake combiner. The space time rate combiner 44 outputs are inputted to a detector 46 which may be VBLAST detector. The outputs of the VBLAST detector 46 are applied to a multiplexer 48 which outputs multiplexed data.
Inter-frequency operation involves the hand-off of the user UE 20′ from one frequency band to another frequency band within a frequency allocation of the system in which the UE is currently registered. Inter-frequency hand-offs may be made for diverse reasons, such as loading of the channels, error rates associated with the transmission of the data, etc.
Inter-system hand-offs involve the hand-off of the UE 20′ from a frequency band in one system in which the UE is currently registered to a frequency band in another system in which the UE is not currently registered.
While the MIMO system 10 of the prior art has improved performance in comparison to a non-MIMO receiver, in order to provide reliable hand-offs to inter-frequencies or inter-system hand-offs, it is necessary for the system 10 to obtain measurements from the UE 20′ which enable the BSC/RNC 14 to make a determination of whether or not a hand-off should be made from the current frequency band in which the UE 20 is operating to another frequency band and furthermore, if such a handoff is to be made, whether it should be inter-frequency hand-off within the same system, e.g. WCDMA or GSM, or between systems e.g., WCDMA to GSM or GSM to WCDMA.